In the very simple process of magnesium combining with oxygen we can see the change in oxidation number happening:

2 Mg + O₂ → 2 MgO
              0       0            +2    -2       

When the oxidation numbers change, the reaction is redox. Contrast that to a precipitation reaction:

MgCl₂ + 2 NaOH → Mg(OH)₂ + 2 NaCl
+2   -1      +1  -2  +1      +2  -2  +1        +1   -1

All of the oxidation numbers remain the same on both sides, so this is not a redox process.